Control of monomeric Vo’s versus Vo clusters in ZrO2−x for solar-light H2 production from H2O at high-yield (millimoles gr−1 h−1)

Pristine zirconia, ZrO2, possesses high premise as photocatalyst due to its conduction band energy edge. However, its high energy-gap is prohibitive for photoactivation by solar-light. Currently, it is unclear how solar-active zirconia can be designed to meet the requirements for high photocatalytic performance. Moreover, transferring this design to an industrial-scale process is a forward-looking route. Herein, we have developed a novel Flame Spray Pyrolysis process for generating solar-light active nano-ZrO2−x via engineering of lattice vacancies, Vo. Using solar photons, our optimal nano-ZrO2−x can achieve milestone H2-production yield, > 2400 μmolg−1 h−1 (closest thus, so far, to high photocatalytic water splitting performance benchmarks). Visible light can be also exploited by nano-ZrO2−x at a high yield via a two-photon process. Control of monomeric Vo versus clusters of Vo’s is the key parameter toward Highly-Performing-Photocatalytic ZrO2−x. Thus, the reusable and sustainable ZrO2−x catalyst achieves so far unattainable solar activated photocatalysis, under large scale production.

www.nature.com/scientificreports/ photocatalytic performances, despite improvement, remain by far inferior versus benchmark photocatalysts, such as TiO 2 which routinely achieves H 2 photoproduction of the order of several (millimoles H 2 g −1 h −1 ) in typical lab set ups 24 . So far, the only ZrO 2 -based photocatalyst passing the threshold of (millimoles g −1 h −1 ) is 2.12 mmol H 2 g −1 h −1 by a N-doped ZrO 2 25 . Without heteroatom doping, generation of reduced states in reducible metal-oxides, such as TiO 2 , are decisively beneficial for H 2 photo yield. Examples include the work of Mao et al. 23 , Naldoni et al. 26 and our Flame Spray Pyrolysis (FSP)-made black TiO 2−x 27 . ZrO 2 is a notoriously non-reducible oxide 28,29 , since introduction of Vo's into the ZrO 2 lattice is not favored energetically 8,[15][16][17][18][19][20] , thus specific synthesis methods are needed to achieve reduction of the ZrO 2 lattice. To this direction, the most significant performance has been reported by Sinhamahapatra et al. 22 and Zu et al. 30 , where defect-rich ZrO 2−x has achieved production of ~ 0.5 mmolg −1 h −1 H 2 in both cases, however, this is still significantly below that of TiO 2 26,31 . Regarding material synthesis, none of the so far reported ZrO 2−x synthesis methods were designed to be scalable at industrial level. Specifically, previous ZrO 2−x synthesis methods include sol-gel 19 , hydrothermal 20 , high pressure/temperature processing 18,21 . More efficient methods able to overcome the non-reducibility of ZrO 2 are magnesiothermic 22 , titaniothermic 23 and lithiothermic reduction 30 where an elementary heterometal M 0 atom, i.e. Li 0 , Ti 0 , Mg 0 , is used to reduce ZrO 2 and create the desired Vo at high yield. However, all aforementioned synthesis routes include multiple steps and do not always allow facile/reproducible control or tailoring of Vo placement and populations. Particularly, the methods which require heterometal contact on ZrO 2 surface e.g. magnesiothermic, rely on harsh acid washes for removing the leftover hetero-metal oxide, something that mounts questions on how this may impact the state of the catalyst itself 22 .
Herein we have developed a one-step Flame Spray Pyrolysis (FSP) process for synthesis of solar-light active nano-ZrO 2−x via engineering of lattice-vacancies, Vo. FSP is eminently suited for synthesis of high crystallinity nano-ZrO 2 32,33 , however, the synthesis of ZrO 2−x has not been reported by FSP. In principle, ZrO 2 can possess two families of reduced states: (i) reduced Zr 3+ centers, and (ii) oxygen vacancies not-located on Zr atoms (Vo's). Over last decades, Giamello's group 34 has provided valuable insights into the complexity of these reduced states. Using Electron Paramagnetic Resonance (EPR) spectroscopy, in combination with quantum chemical calculations 34 , they prove that Zr 3+ centers can create extra energy states right below the E CB of pristine ZrO 2 , (~ 4.5-5.0 eV). Based on all existing evidence, these Zr 3+ centers are expected to have little effect on the photocatalytic activity of zirconia 35 . On the other hand, Vo's can create mid-gap states 34 but their role in photocatalytic H 2 evolution has not been explored systematically. Herein, using FSP we have produced libraries of ZrO 2−x nano catalysts with varying concentrations of Vo's and identified the optimal configuration, towards high photocatalytic H 2 -production efficiency from H 2 O. Specific aims of the present work were: (i) to develop a novel industrial-scale FSP method for one-step synthesis of nanosized ZrO 2−x with controllable population and placement of the O-vacancies (Vo's). No heteroatoms were used. (ii) to optimize the ZrO 2−x for highly efficient solar light H 2 production, well beyond the current state of the art, i.e., well-above the threshold mmol g −1 h −1 , (iii) to provide a comprehensive understanding of the physicochemical role of Vo, with emphasis on monomeric versus clusters of Vo's related to the photocatalytic properties. We present a novel anoxic-FSP process that allows in-situ formation of Vo's during the primary particle formation step. Using solar photons, the optimal nano-ZrO 2−x can achieve milestone H 2 -production yield, > 2400 μmolg −1 h −1 which is the closest so far to high photocatalytic performance benchmarks. We demonstrate that optimal nano-ZrO 2−x can be achieved by controlling the monomeric Vo versus clusters of Vo's by two routes either during the FSP synthesis or via a short post-FSP oxidation process.

Results
Synthesis of nano ZrO 2−x by anoxic-flame spray pyrolysis. The concept of the novel anoxic-FSP process is outlined in Fig. 1. It consists of a single-nozzle FSP reactor with enclosed flame, where a mixture of Dispersion-gas [O 2 and CH 4 ] is used to create a reducing reaction atmosphere. In FSP-process 35 , ZrO 2 particles are formed in three stages (Fig. 1). First, Zr-precursor droplets are sprayed by the FSP nozzle and combusted to generate the primary particles (PP) 35 . Then, primary particles evolve in the high-temperature area of the flame, i.e. up to 2800 K and form nanometric ZrO 2 particles via sintering of PP's 35 . In classical-FSP, used in the majority of lab studies and industry, pure O 2 is used as dispersion gas through the spray nozzle, to form the droplets and primary particles. For example, by adjusting the combustion stoichiometry 35 ratio P/D = [fuel/dispersion O 2 ] = 3/3, we obtain fully oxidized, pristine ZrO 2 . In our anoxic-FSP, combustion of CH 4 in the dispersion gas creates reducing agents which, as we show herein, can reduce the primary Zr-particle via formation of Oxygen vacancies (Vo). We also have considered the possibility of the formation of Zr-Hydride states 36 , however, none of our data support this, thus we do not discuss it further.
In this way, we have produced a library of ZrO 2−x nanoparticles of varying Vo concentrations, see photos in Fig. 2 (Fig. 2d) shows a progressive increase of Vo's, detected by their characteristic signal at 532 eV 37,38 . We have verified that no-carbon deposition is evidenced by Raman data, (Supplementary Fig. S1), thus the observed color changes in the ZrO 2−x materials (Fig. 2), are assigned exclusively to the formation of Vo's via the anoxic-FSP process. According to XPS, Fig. 2d  www.nature.com/scientificreports/    (Table S2) shows progressive SSA-decrease upon increase dispersion-CH 4 , attributed to increased aggregation of the particles at increased CH 4 , i.e. methane creates hotter flames i.e. methane heat of combustion = 50-55 MJ/kg (https:// webbo ok. nist. gov/ chemi stry/). Raman spectra (Fig. 4b) exhibit the vibrational modes from both monoclinic and tetragonal crystal phases 40-43 ( Fig. S4 and Table S3) and absence of carbon peaks (Fig. S1). In ZrO 2−x materials, certain Raman modes are shifted (see Fig. 4b(I-III) and Table S4). More particularly, material [3/0.1] exhibits upshift of + 4 cm −1 at 313 cm −1 mode (Table S4) 44 . Accordingly, the present Raman data indicate that removal of oxygen from the ZrO 2 matrices is also non-equivalent, thus is easier to extract oxygen from an O 4f site rather than an O 3f site by 0.1 eV 22 , therefore it is easier to reduce t-ZrO 2 .
Photocatalytic H 2 production at (millimole gr −1 h −1 ). Figure 5a presents the photocatalytic H 2 production from H 2 O, for all our ZrO 2−x materials, under Xenon-illumination. In each panel in Fig. 5a the as-prepared photocatalysts are marked as "a.p". The time indication in each bar, refers to the post-FSP oxidation-time at 400 °C (see also XRD data in Fig. S3 in S.I.). First, we discuss the as-prepared ZrO 2−x materials i.e., see the first bar in each column group in Fig. 5a. Pristine, (F.O.) ZrO 2 was practically non-photoactive in H 2 production, with a yield of 20 μmol g −1 h −1 .
In contrast, a slightly reducing FSP atmosphere, i.e., as-prepared [a.p. 3/0.1], enables an impressive amelioration of H 2 evolution of 1700 μmol g −1 h −1 . This demonstrates that anoxic-FSP can provide as-prepared ZrO 2−x material exhibiting millimoles per gram per hour H 2 production. Further increase of O 2 /CH 4 ratio impacted negatively the H 2 photogeneration with a tendency towards a steady production near 500 μmol g −1 h −1 of H 2 for www.nature.com/scientificreports/ the highly reduced as-prepared materials (see the first bar in each group in Fig. 5a (Fig. 5a). Table S5 summarizes a comparison of FSP-ZrO 2−x versus other pertinent ZrO 2−x materials reported in the literature (see also Fig. S7). The catalyst with the higher H 2 yield, [2.3/0.7]-90, is highly recyclable, (Fig. S5a), retaining 100% of its activity after two reuses and > 96% after four reuses. XRD (Fig. S5b), shows that the [2.3/0.7]-90 crystal remains intact after 4-uses. Concurrently, DRS-UV/Vis (Fig. S5c), demonstrates that its light-absorbance profile remains also intact. As we discuss hereafter, optimization of monomeric Vo-concentration is determinant for photocatalytic activity. In [2.3/0.7]-90, the monomeric Vo's is optimized, see EPR and XPS data in Figs. 5d,e, and S6a,b see also the trends in XPS, EPR for [2.3/0.7] in Fig. 5d,e. After photocatalytic use of [2.3/0.7]-90 material, the monomeric vacancies are not altered, neither the ZrO 2−x crystal. Thus, the ZrO 2−x provide a robust reusable photocatalyst.

Discussion
The data in Fig. 5a demonstrate that there are two options to achieve high-photocatalytic performance ZrO 2−x materials: either (i) to be prepared at low O 2 /CH 4 ratio e.g.  Fig. 6c. Oxidation progressively eliminates Vo clusters towards monomeric Vo's. This is also evident from the progressive elimination of the deep-grey color, and the changes in the UV-Vis spectra (Fig. S8 in SI). Taking this information into account, the bell-shaped H 2 production trend in Fig. 5b, indicates that a high concentration of O-vacancies, forming Vo clusters, is detrimental to the photocatalytic activity of ZrO 2−x . Fewer Vo's are better suited for optimal photocatalytic activity (see trend in Fig. 5a, for [2.3/0.7]). However, further oxidation of Vo's tends to delimit the photoactivity. This teaches us that a quantitative control of the Vo's clusters versus monomeric Vo is necessary, to achieve highly-performance photocatalytic ZrO 2−x , see full trend in Fig. 6e. We consider that this factor was also of pertinence in the magnesiothermically reduced ZrO 2−x materials 22 . Although not noticed by these authors 22 , inspection of their EPR spectra, shows that these correspond to Vo clusters, which concurs with their limited H 2 production of 506 μmol H 2 g −1 h −1 , resembling our as-prepared [2.3/0.7] material.
The data under visible 405 nm LED irradiation (Fig. 5c), indicate that significant part of photocatalytic H 2 production, at least 70% versus the solar-light photons, can be excited by visible 405 nm photons (3.1 eV). Taking into account the DRS-UV/Vis data, (Fig. 2c) this can be attributed to the occurrence of the mid-gap states, (Fig. 5f). Theoretical DFT calculations (Fig. S10) show that in ZrO 2−x , few oxygen vacancies can create mid-gap states, located at energy distances around 3.0 eV 12,45 from both the VB-top and ~ 2.0 eV from the CB-bottom. Thus, in ZrO 2−x the 3.1 eV photons (405 nm LED) are able to excite two consecutive electron transitions, (Fig. 5f). Increased anoxicity, i.e., as in material [2.3/0.7], enhances the DOS band-tailing (see S.I. Figs. S10 and 5f). This would increase the probability of two-photon electron photoexcitation via VB → Vo, and Vo → CB. These electrons are favorably transferred to the Pt particles, which act as electron collectors i.e. work function of Pt, φ = + 0.9 eV versus NHE 46 , is favorable for acceptance of electrons from the highly-excited electrons in the CB. Fig. 6a, show that monomeric Vo's were characterized by an inhomogeneous lineshape with linewidth �H monomer = (9.4 ± 0.1)Gauss and a rhombic g monomer -tensor, (Table S5). Vo clusters are characterized by a Lorenz line-shape and �H cluster = (4.6 ± 0.1) Gauss and isotropic g cluster -tensor, (Table S6).

Quantitate analysis of V O -clusters versus monomeric Vo's, by EPR and XPS. Numerical EPR simulations, dashed lines
The structural significance of this is: for isotropic EPR signals with low g-anisotropy, a Gaussian line-shape is the fingerprint of the so-called inhomogeneously-broadened S = 1/2 states 47,48 which is indicative of isolated Vo's, with no-interactions 47,48 . Physically, this indicates that in the ZrO 2−x particles produced under low CH 4 -flow e.g. [3/0.1], monomeric isolated Vo EPR centers. The Lorentz line-shape of the Vo clusters indicates that it originates from Vo centers with spin-exchange and/or fast dipolar interactions. In ZrO 2−x , this Lorentz line-shape indicates formation of Vo-clusters upon increasing dispersion-CH 4 .
Comparing the XPS data for surface-Vo's ( Fig. 6d) versus the quantitative data for total-paramagnetic Vo's, (Fig. 6c and Table S7 in SI), derived from the EPR spectra (Fig. 6b) and their deconvolution in monometer/ clusters (see example for [1.3/1.7] in Fig. S9 in SI), we notice a correlation: highly reduced ZrO 2−x , have higher surface-Vo's, and total paramagnetic Vo's. Figure 6e provides an overview plot, which shows that optimization of H 2 photocatalysis can be achieved via optimization of V O -Clusters versus monomeric Vo's by two routes: (i) control of FSP anoxicity, or (ii) by soft post-FSP oxidation. This demonstrates that our novel anoxic-FSP process allows facile synthesis of solar-light active nano-ZrO 2−x via engineering of lattice-vacancies, Vo. Control of monomeric Vo versus clusters of Vo's is the key-parameter toward Highly-Performing-Photocatalytic ZrO 2−x . The anoxic-FSP process presented here, should be easily adaptable to existing industrial-scale FSP reactors. This offers an efficient technology that can be adopted in the future and provide new tools for the design of other families of photoactive nanomaterials via control of oxygen vacancies.  Then, the solution was fed through a capillary at 3 ml min −1 and dispersed to a self-sustained oxygen/methane (4-2 L min −1 ) pilot flame to initiate combustion. An important distinction must be made which leads to the innovation of the presented work. ZrO 2−x materials were prepared by modifying the dispersion feed. While keeping the 3 ml min −1 dispersion constant, methane gas (CH 4 ) was fed along with the traditional dispersion gas (O 2 ). The resulting high temperatures and hydride formation through the decomposition of methane lead to the formation of ZrO 2−x . Furthermore, the protocol of methane injection ensures the formation of bulk defects as the particle is influenced at its early stages of flight, at the primary particle stage. Finally, the pressure drop was fixed at 1.5-2.0 bar, and an additional 10 L min −1 O 2 sheath was used to aid in particle collection which was made possible by a vacuum pump (Busch V40) and by a glass microfiber filter (GF 6 257, Hahnemühle, Dassel, Germany). www.nature.com/scientificreports/ Characterization techniques. Powder X-Ray Diffraction (XRD) data were collected at room temperature using a Bruker D8 Advance 2theta diffractometer with copper radiation (Cu Ka, λ = 1.5406 Å) and a secondary monochromator operating at 36 kV and 36 mA. Crystal size is calculated by the Scherrer formula. X-Ray photoelectron spectroscopy (XPS) data were collected by a surface analysis ultrahigh vacuum system (SPECS GmbH) equipped with a twin Al-Mg anode X-ray source and a multichannel hemispherical sector electron analyzer (HSA Phoibos 100). The base pressure was 2 − 5 × 10 −9 mbar. A monochromatized Mg Kα line at 1253.6 eV and analyzer pass energy of 20 eV were used in all XPS measurements. The binding energies were calculated with reference to the energy of C 1s peak of contaminant carbon at 284.5 eV. The peak deconvolution was calculated using a Shirley background. Raman HORIBA-Xplora Plus spectrometer, equipped with an Olympus BX41 microscope. A 785 nm diode laser was used as an excitation source, and the laser beam was focused on the sample with the aid of the microscope. Before measurement, each powder material was softly pressed between two glass plates to form a pelletlike structure. Brunauer-Emmett-Teller (BET) adsorption-desorption isotherms were recorded at 77 K using a Quantachrome NOVA touch LX 2 . Outgassing was performed at 80 °C for 5 h under vacuum, before the measurements. The absorption data points in the relative pressure P/P o range of 0.1-0.3 was used to calculate the specific surface area (SSA).
Electron paramagnetic resonance spectroscopy. The X-band electron paramagnetic resonance (EPR) spectra of ZrO 2 /ZrO 2−x materials were recorded with a Bruker ER200D spectrometer at 77 K, equipped with an Agilent 5310A frequency counter. The spectrometer was running under a custom-made software based on LabView. Adequate signal-to-noise ratio was obtained after 15-20 scans, with a microwave power fixed at 20 mW. The EPR instrumental conditions were as follows: microwave frequency = 9.53 GHz and modulation amplitude = 10 Gpp.
Theoretical analysis of the EPR spectra. The experimental EPR spectra were simulated using the EasySpin software. A S = 1/2 Spin Hamiltonian was used Ĥ = β � B · g · � S where β is the Bohr magneton, B is the applied magnetic field, g is the spectroscopic g-tensor and S the spin angular momentum. The X-band electron paramagnetic resonance (EPR) spectra were recorded with a Bruker ER200D spectrometer at 77 K, equipped with an Agilent 5310A frequency counter. The spectrometer was running under a home-made software based on LabView. Adequate signal-to-noise ratio was obtained after 30-50 scans. The EPR instrumental conditions were as follows: microwave frequency = 9.55 GHz, modulation frequency = 50.00 kHz, and modulation amplitude = 10 Gauss peak-to-peak.
Photocatalytic H 2 evolution procedure. The photocatalytic hydrogen reactions were realized into a double wall Pyrex reactor, cooled with tap circulation (T = 25 °C). Light source was a Solar Simulator, (Sciencetech, Class AAA, model SciSun-150) with average irradiation intensity of 180 W m −2 equipped with a xenon lamp of 150 W and Air Mass filter (1 sun, AM1.5G). As Visible light source was used a Led lamp FireEdge™ FE410 (λ = 405 nm) supplied by Phoseon company, which power intensity was set to be 180 W m −2 , using a power meter (Thorlabs Inc., USA). In each experiment, 50 mg of the catalyst was suspended into 150 ml water/methanol mixture 20% v/v (final concentration of the catalyst 330 mg L −1 ). Atmospheric O 2 from the suspension was removed, fulfilling the content of reactor with Ar gas (99.9997%) at least 1 h. As Pt source were used the dihydrogen hexachloroplatinate (IV) hydrate complex (H 2 Pt 4 Cl 6 . 6H 2 O, 99.99%, Αlfa Αesar) which was photodeposited in situ, at the reaction mixture. Qualitative and quantitative monitoring of produced H 2 and CO 2 gases was done via a continuous online GasChromatography System combined with a Thermo-conductive Detector (GC-TCD-Shimadzu GC-2014, carboxen 1000 column, Ar carrier gas 49 .
Post-FSP oxidation. A ThermaWatt furnace was used, equipped with a tubular Quartz compartment 50 .
Oxidations were performed under atmospheric O 2 at temperature 400 °C and calcination time was varied from 30 to 120 min through intervals of 30 min.

Data availability
All data generated or analysed during this study are included in this published article (and its supplementary information files).